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March 3, 2009

Social Class Modifies Effect of BMI on Breast Cancer Risk

Higher social class and heavier body size are known risk factors for breast cancer. While these factors primarily work independently, for postmenopausal women social resources may moderate the influence of body size on breast cancer risk, according to a study led by researchers from the Johns Hopkins Bloomberg School of Public Health. The study is among the first to examine the interaction between social class and body size on breast cancer risk. The findings have been published electronically by the American Journal of Public Health and will appear in the April 2009 print edition.

“Scientists have long recognized that women of higher social class are at greater risk of breast cancer, in part because they tend to have fewer children and have them later in life. Our study hints that something else may be going on, because reproductive factors do not account for all of the risk,” said lead author Celeste Marie Torio, PhD. Torio conducted the research as a doctoral candidate at the Bloomberg School of Public Health and is now with the Robert Wood Johnson Foundation.

For the study, Torio and colleagues analyzed data from 5,634 postmenopausal white women recruited to CLUE II, an ongoing perspective cohort study of residents of Washington County, Md., maintained by the George W. Comstock Center for Public Health Research and Prevention. Consistent with previous research, they found, after adjusting for age, that a woman’s own level of education remained a significant predictor of increased breast cancer risk, and that both current body mass index (BMI) as well as weight gain since age 21,  were also significant predictors of breast cancer risk. 

After accounting for a woman’s own level of formal education, the researchers found that contextual factors, including social class and access to material resources, did not have an additional direct influence on breast cancer risk. However, there was a significant protective interaction between area-level measures of social class and current weight, as well as area-level social class and weight gain since age 21.

“This suggests that a woman’s current social environment can reduce the negative influences of excess body size on breast cancer risk, and, conversely, that heavier women in low- resource environments may face additional risk for breast cancer. Our study adds to the evidence that the lifelong effects of education achieved as a younger woman, as well as the social resources shaping a woman’s life currently, are both important influences on older women’s health,” said Ann C. Klassen, PhD, co-author of the study and associate professor in the Bloomberg School’s Department of Health, Behavior and Society.

Additional authors of “The Modifying Effect of Social Class on the Relationship between Body Mass Index and Breast Cancer” are Frank C. Curriero, PhD; Benjamin Caballero, MD, PhD; and Kathy Helzlsouer, MD, MHS.

The research was funded in part by the National Cancer Institute of the National Institutes of Health and the Department of Defense.